Regulation of PIP5K activity by Arf6 and its physiological significance

The Role of KACh Channels in Atrial Fibrillation

This manuscript explores the intricate role of acetylcholine-activated inward rectifier potassium (KACh) channels in the pathogenesis of atrial fibrillation (AF), a common cardiac arrhythmia. It delves into the molecular and cellular mechanisms that underpin AF, emphasizing the vital function of KACh channels in modulating the atrial action potential and facilitating arrhythmogenic conditions. This study underscores the dual nature of KACh activation and its genetic regulation, revealing that specific variations in potassium channel genes, such as Kir3.4 and K2P3.1, significantly influence the electrophysiological remodeling associated with AF. Furthermore, this manuscript identifies the crucial role of the KACh-mediated current, IKACh, in sustaining arrhythmia through facilitating shorter re-entry circuits and stabilizing the re-entrant circuits, particularly in response to vagal nerve stimulation. Experimental findings from animal models, which could not induce AF in the absence of muscarinic activation, highlight the dependency of AF induction on KACh channel activity. This is complemented by discussions on therapeutic interventions, where KACh channel blockers have shown promise in AF management. Additionally, this study discusses the broader implications of KACh channel behavior, including its ubiquitous presence across different cardiac regions and species, contributing to a comprehensive understanding of AF dynamics. The implications of these findings are profound, suggesting that targeting KACh channels might offer new therapeutic avenues for AF treatment, particularly in cases resistant to conventional approaches. By integrating genetic, cellular, and pharmacological perspectives, this manuscript offers a holistic view of the potential mechanisms and therapeutic targets in AF, making a significant contribution to the field of cardiac arrhythmia research.

Pip5k1γ promotes anabolism of nucleus pulposus cells and intervertebral disc homeostasis by activating CaMKII-Ampk pathway in aged mice

Degenerative disc disease (DDD) represents a significant global health challenge, yet its underlying molecular mechanisms remain elusive. This study aimed to investigate the role of type 1 phosphatidylinositol 4-phosphate 5-kinase (Pip5k1) in intervertebral disc (IVD) homeostasis and disease. All three Pip5k1 isoforms, namely Pip5k1α, Pip5k1β, and Pip5k1γ, were detectable in mouse and human IVD tissues, with Pip5k1γ displaying a highest expression in nucleus pulposus (NP) cells. The expression of Pip5k1γ was significantly down-regulated in the NP cells of aged mice and patients with severe DDD. To determine whether Pip5k1γ expression is required for disc homeostasis, we generated a Pip5k1γfl/fl; AggrecanCreERT2 mouse model for the conditional knockout of the Pip5k1γ gene in aggrecan-expressing IVD cells. Our findings revealed that the conditional deletion of Pip5k1γ did not affect the disc structure or cellular composition in 5-month-old adult mice. However, in aged (15-month-old) mice, this deletion led to several severe degenerative disc defects, including decreased NP cellularity, spontaneous fibrosis and cleft formation, and a loss of the boundary between NP and annulus fibrosus. At the molecular level, the absence of Pip5k1γ reduced the anabolism of NP cells without markedly affecting their catabolic or anti-catabolic activities. Moreover, the loss of Pip5k1γ significantly dampened the activation of the protective Ampk pathway in NP cells, thereby accelerating NP cell senescence. Notably, Pip5k1γ deficiency blunted the effectiveness of metformin, a potent Ampk activator, in activating the Ampk pathway and mitigating lumbar spine instability (LSI)-induced disc lesions in mice. Overall, our study unveils a novel role for Pip5k1γ in promoting anabolism and maintaining disc homeostasis, suggesting it as a potential therapeutic target for DDD.

Evidence for Involvement of ADP-Ribosylation Factor 6 in Intracellular Trafficking and Release of Murine Leukemia Virus Gag

Murine leukemia viruses (MuLVs) are simple retroviruses that cause several diseases in mice. Retroviruses encode three basic genes: gag, pol, and env. Gag is translated as a polyprotein and moves to assembly sites where viral particles are shaped by cleavage of poly-Gag. Viral release depends on the intracellular trafficking of viral proteins, which is determined by both viral and cellular factors. ADP-ribosylation factor 6 (Arf6) is a small GTPase that regulates vesicular trafficking and recycling of different types of cargo in cells. Arf6 also activates phospholipase D (PLD) and phosphatidylinositol-4-phosphate 5-kinase (PIP5K) and produces phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2). We investigated how Arf6 affected MuLV release with a constitutively active form of Arf6, Arf6Q67L. Expression of Arf6Q67L impaired Gag release by accumulating Gag at PI(4,5)P2-enriched compartments in the cytoplasm. Treatment of the inhibitors for PLD and PIP5K impaired or recovered MuLV Gag release in the cells expressing GFP (control) and Arf6Q67L, implying that regulation of PI(4,5)P2 through PLD and PIP5K affected MuLV release. Interference with the phosphoinositide 3-kinases, mammalian target of rapamycin (mTOR) pathway, and vacuolar-type ATPase activities showed further impairment of Gag release from the cells expressing Arf6Q67L. In contrast, mTOR inhibition increased Gag release in the control cells. The proteasome inhibitors reduced viral release in the cells regardless of Arf6Q67L expression. These data outline the differences in MuLV release under the controlled and overactivated Arf6 conditions and provide new insight into pathways for MuLV release.

Transcriptome Analysis Reveals the Involvement of Mitophagy and Peroxisome in the Resistance to QoIs in Corynespora cassiicola

Quinone outside inhibitor fungicides (QoIs) are crucial fungicides for controlling plant diseases, but resistance, mainly caused by G143A, has been widely reported with the high and widespread use of QoIs. However, two phenotypes of Corynespora casiicola (RI and RII) with the same G143A showed significantly different resistance to QoIs in our previous study, which did not match the reported mechanisms. Therefore, transcriptome analysis of RI and RII strains after trifloxystrobin treatment was used to explore the new resistance mechanism in this study. The results show that 332 differentially expressed genes (DEGs) were significantly up-regulated and 448 DEGs were significantly down-regulated. The results of GO and KEGG enrichment showed that DEGs were most enriched in ribosomes, while also having enrichment in peroxide, endocytosis, the lysosome, autophagy, and mitophagy. In particular, mitophagy and peroxisome have been reported in medicine as the main mechanisms of reactive oxygen species (ROS) scavenging, while the lysosome and endocytosis are an important organelle and physiological process, respectively, that assist mitophagy. The oxidative stress experiments showed that the oxidative stress resistance of the RII strains was significantly higher than that of the RI strains: specifically, it was more than 1.8-fold higher at a concentration of 0.12% H2O2. This indicates that there is indeed a significant difference in the scavenging capacity of ROS between the two phenotypic strains. Therefore, we suggest that QoIs' action caused a high production of ROS, and that scavenging mechanisms such as mitophagy and peroxisomes functioned in RII strains to prevent oxidative stress, whereas RI strains were less capable of resisting oxidative stress, resulting in different resistance to QoIs. In this study, it was first revealed that mitophagy and peroxisome mechanisms available for ROS scavenging are involved in the resistance of pathogens to fungicides.

De novo missense variants in phosphatidylinositol kinase PIP5KIγ underlie a neurodevelopmental syndrome associated with altered phosphoinositide signaling

Phosphoinositides (PIs) are membrane phospholipids produced through the local activity of PI kinases and phosphatases that selectively add or remove phosphate groups from the inositol head group. PIs control membrane composition and play key roles in many cellular processes including actin dynamics, endosomal trafficking, autophagy, and nuclear functions. Mutations in phosphatidylinositol 4,5 bisphosphate [PI(4,5)P2] phosphatases cause a broad spectrum of neurodevelopmental disorders such as Lowe and Joubert syndromes and congenital muscular dystrophy with cataracts and intellectual disability, which are thus associated with increased levels of PI(4,5)P2. Here, we describe a neurodevelopmental disorder associated with an increase in the production of PI(4,5)P2 and with PI-signaling dysfunction. We identified three de novo heterozygous missense variants in PIP5K1C, which encodes an isoform of the phosphatidylinositol 4-phosphate 5-kinase (PIP5KIγ), in nine unrelated children exhibiting intellectual disability, developmental delay, acquired microcephaly, seizures, visual abnormalities, and dysmorphic features. We provide evidence that the PIP5K1C variants result in an increase of the endosomal PI(4,5)P2 pool, giving rise to ectopic recruitment of filamentous actin at early endosomes (EEs) that in turn causes dysfunction in EE trafficking. In addition, we generated an in vivo zebrafish model that recapitulates the disorder we describe with developmental defects affecting the forebrain, including the eyes, as well as craniofacial abnormalities, further demonstrating the pathogenic effect of the PIP5K1C variants.

The Arf family GTPases: Regulation of vesicle biogenesis and beyond

The Arf family proteins are best known for their roles in the vesicle biogenesis. However, they also play fundamental roles in a wide range of cellular regulation besides vesicular trafficking, such as modulation of lipid metabolic enzymes, cytoskeleton remodeling, ciliogenesis, lysosomal, and mitochondrial morphology and functions. Growing studies continue to expand the downstream effector landscape of Arf proteins, especially for the less-studied members, revealing new biological functions, such as amino acid sensing. Experiments with cutting-edge technologies and in vivo functional studies in the last decade help to provide a more comprehensive view of Arf family functions. In this review, we summarize the cellular functions that are regulated by at least two different Arf members with an emphasis on those beyond vesicle biogenesis.

Extracellular vesicles in β cell biology: Role of lipids in vesicle biogenesis, cargo, and intercellular signaling

BACKGROUND

Type 1 diabetes (T1D) is a complex autoimmune disorder whose pathogenesis involves an intricate interplay between β cells of the pancreatic islet, other islet cells, and cells of the immune system. Direct intercellular communication within the islet occurs via cell surface proteins and indirect intercellular communication has traditionally been seen as occurring via secreted proteins (e.g., endocrine hormones and cytokines). However, recent literature suggests that extracellular vesicles (EVs) secreted by β cells constitute an additional and biologically important mechanism for transmitting signals to within the islet.

SCOPE OF REVIEW

This review summarizes the general mechanisms of EV formation, with a particular focus on how lipids and lipid signaling pathways influence their formation and cargo. We review the implications of EV release from β cells for T1D pathogenesis, how EVs and their cargo might be leveraged as biomarkers of this process, and how EVs might be engineered as a therapeutic candidate to counter T1D outcomes.

MAJOR CONCLUSIONS

Islet β cells have been viewed as initiators and propagators of the cellular circuit giving rise to autoimmunity in T1D. In this context, emerging literature suggests that EVs may represent a conduit for communication that holds more comprehensive messaging about the β cells from which they arise. As the field of EV biology advances, it opens the possibility that intervening with EV formation and cargo loading could be a novel disease-modifying approach in T1D.

ADP-ribosylation factor 6 expression increase in oesophageal adenocarcinoma suggests a potential biomarker role for it

ADP-ribosylation factor 6 small GTPase plays an important role in cell migration, invasion and angiogenesis, which are the hallmarks of cancer. Although alterations in ARF6 expression and activity have been linked to metastatic cancer in one or two tissues, the expression of ARF6 in cancers over a wide range of tissues has not been studied so far. In this report, we analysed the expression of ARF6 mRNA in cancers and corresponding healthy controls from 17 different tissues by real-time qualitative polymerase chain reaction (RT-qPCR). We further evaluated ARF6 protein expression in oesophageal adenocarcinoma (EAC) tissue microarray cores by immunohistochemistry. The ARF6 gene expression levels are highly variable between healthy and cancer tissues. Our findings suggest that the ARF6 gene expression is up-regulated highest in oesophageal cancer. In EAC TMAs, ARF6 protein expression increase correlated with EAC progression. This is the first study to investigate ARF6 gene expression in a wide array of cancer tissues and demonstrate that ARF6 expression, at both mRNA and protein levels, is significantly upregulated in higher grades of EAC, which may be useful in targeting ARF6 for cancer diagnostic and therapeutic purposes.

Pip5k1c Loss in Chondrocytes Causes Spontaneous Osteoarthritic Lesions in Aged Mice

Osteoarthritis (OA) is the most common degenerative joint disease affecting the older populations globally. Phosphatidylinositol-4-phosphate 5-kinase type-1 gamma (Pip5k1c), a lipid kinase catalyzing the synthesis of phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2), is involved in various cellular processes, such as focal adhesion (FA) formation, cell migration, and cellular signal transduction. However, whether Pip5k1c plays a role in the pathogenesis of OA remains unclear. Here we show that inducible deletion of Pip5k1c in aggrecan-expressing chondrocytes (cKO) causes multiple spontaneous OA-like lesions, including cartilage degradation, surface fissures, subchondral sclerosis, meniscus deformation, synovial hyperplasia, and osteophyte formation in aged (15-month-old) mice, but not in adult (7-month-old) mice. Pip5k1c loss promotes extracellular matrix (ECM) degradation, chondrocyte hypertrophy and apoptosis, and inhibits chondrocyte proliferation in the articular cartilage of aged mice. Pip5k1c loss dramatically downregulates the expressions of several key FA proteins, including activated integrin β1, talin, and vinculin, and thus impairs the chondrocyte adhesion and spreading on ECM. Collectively, these findings suggest that Pip5k1c expression in chondrocytes plays a critical role in maintaining articular cartilage homeostasis and protecting against age-related OA.

A Eukaryote-Wide Perspective on the Diversity and Evolution of the ARF GTPase Protein Family

The evolution of eukaryotic cellular complexity is interwoven with the extensive diversification of many protein families. One key family is the ARF GTPases that act in eukaryote-specific processes, including membrane traffic, tubulin assembly, actin dynamics, and cilia-related functions. Unfortunately, our understanding of the evolution of this family is limited. Sampling an extensive set of available genome and transcriptome sequences, we have assembled a data set of over 2,000 manually curated ARF family genes from 114 eukaryotic species, including many deeply diverged protist lineages, and carried out comprehensive molecular phylogenetic analyses. These reconstructed as many as 16 ARF family members present in the last eukaryotic common ancestor, nearly doubling the previously inferred ancient system complexity. Evidence for the wide occurrence and ancestral origin of Arf6, Arl13, and Arl16 is presented for the first time. Moreover, Arl17, Arl18, and SarB, newly described here, are absent from well-studied model organisms and as a result their function(s) remain unknown. Analyses of our data set revealed a previously unsuspected diversity of membrane association modes and domain architectures within the ARF family. We detail the step-wise expansion of the ARF family in the metazoan lineage, including discovery of several new animal-specific family members. Delving back to its earliest evolution in eukaryotes, the resolved relationship observed between the ARF family paralogs sets boundaries for scenarios of vesicle coat origins during eukaryogenesis. Altogether, our work fundamentally broadens the understanding of the diversity and evolution of a protein family underpinning the structural and functional complexity of the eukaryote cells.

Phospholipase D and phosphatidylinositol-4-phosphate 5-kinase 1 are involved in the regulation of oligodendrocyte morphological differentiation

Oligodendroglial cells (oligodendrocytes) differentiate to form the myelin that wraps neuronal axons in the central nervous system (CNS). This myelin sheath supports the propagation of saltatory conduction and protects axons from physical stresses. When oligodendrocytes do not normally differentiate to myelinate axons, their key functions as oligodendrocytes in the CNS are severely impaired. The molecular mechanics that control differentiation still remain to be clarified. Arf6 belongs to the small GTPase family and is known to be a positive regulator of oligodendrocyte differentiation. Here, we show that the phospholipase D (PLD) and phosphatidylinositol-4-phosphate 5-kinase 1 (PIP5K1) molecules, the major effectors of Arf6, are involved in the regulation of oligodendrocyte differentiation. Knockdown of PLD1 or PIP5K type 1γ (PIP5K1C) by their respective specific siRNAs in mouse oligodendroglial FBD-102b cells inhibited morphological differentiation into structures bearing myelin-like processes; this finding is consistent with the concurrent changes in expression of differentiation and myelin marker proteins. Treatment with VU0155069 or UNC3230, specific inhibitors of PLD and PIP5K1, respectively, blunted morphological differentiation and decreased expression of myelin and differentiation marker proteins. Similar results have been obtained in studies using primary oligodendrocytes. These results suggest that the major Arf6 effector molecules PLD and PIP5K1 are among the molecules involved in the regulation of morphological differentiation in oligodendrocytes prior to myelination.

Systems biology reveals reprogramming of the S-nitroso-proteome in the cortical and striatal regions of mice during aging process

Cell aging depends on the rate of cumulative oxidative and nitrosative damage to DNA and proteins. Accumulated data indicate the involvement of protein S-nitrosylation (SNO), the nitric oxide (NO)-mediated posttranslational modification (PTM) of cysteine thiols, in different brain disorders. However, the changes and involvement of SNO in aging including the development of the organism from juvenile to adult state is still unknown. In this study, using the state-of-the-art mass spectrometry technology to identify S-nitrosylated proteins combined with large-scale computational biology, we tested the S-nitroso-proteome in juvenile and adult mice in both cortical and striatal regions. We found reprogramming of the S-nitroso-proteome in adult mice of both cortex and striatum regions. Significant biological processes and protein–protein clusters associated with synaptic and neuronal terms were enriched in adult mice. Extensive quantitative analysis revealed a large set of potentially pathological proteins that were significantly upregulated in adult mice. Our approach, combined with large scale computational biology allowed us to perform a system-level characterization and identification of the key proteins and biological processes that can serve as drug targets for aging and brain disorders in future studies.

Phosphatidylinositol-4-phosphate 5-kinase type 1α attenuates Aβ production by promoting non-amyloidogenic processing of amyloid precursor protein

Alzheimer's disease (AD) is characterized by a chronic decline in cognitive function and is pathologically typified by cerebral deposition of amyloid-β peptide (Aβ). The production of Aβ is mediated by sequential proteolysis of amyloid precursor protein (APP) by β- and γ-secretases, and has been implicated as the essential determinant of AD pathology. Previous studies have demonstrated that the level of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] in the membrane may potentially modulate Aβ production. Given that PI(4,5)P2 is produced by type 1 phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks), we sought to determine whether the level of PIP5K type Iα (PIP5K1A) can affect production of Aβ by modulating the lipid composition of the membrane. Using a HEK-derived cell line that constitutively expresses yellow fluorescent protein-tagged APP (APP-YFP), we demonstrated that overexpression of PIP5K1A results in significant enhancement of non-amyloidogenic APP processing and a concomitant suppression of the amyloidogenic pathway, leading to a marked decrease in secreted Aβ. Consistently, cells overexpressing PIP5K1A exhibited a significant redistribution of APP-YFP from endosomal compartments to the cell surface. Our findings suggest that PIP5K1A may play a critical role in governing Aβ production by modulating membrane distribution of APP, and as such, the pathway may be a valuable therapeutic target for AD.

BIG1 controls macrophage pro-inflammatory responses through ARF3-mediated PI(4,5)P2 synthesis

Sepsis is caused by a dysregulated host inflammatory response to serious infections resulting in life-threatening organ dysfunction. The high morbidity and mortality make sepsis still a major clinical problem. Here, we investigated the roles of Brefeldin A-inhibited guanine nucleotide-exchange factor 1 (BIG1) in the pathogenesis process of sepsis and the underlying mechanisms. We found myeloid cell-specific BIG1 knockout (BIG1 cKO) significantly reduced the mortality and organ damage in LPS-induced and CLP-induced polymicrobial sepsis mouse model. The serum concentration and mRNA expression of pro-inflammatory cytokines including TNF-α, IL-6, IL-1β, and IL-12 were obviously decreased in BIG1 cKO mice. In bone marrow-derived macrophages or THP-1 cells, BIG1 deficiency caused an inhibited ARF3 activation, which reduced PI(4,5)P2 synthesis and the recruitment of TIRAP to the plasma membrane through inhibiting the activation of PIP5K induced by LPS, and eventually resulted in the inhibitory activity of TLR4-MyD88 signaling pathway. These results reveal a crucial new role of BIG1 in regulating macrophage inflammation responses, and provide evidence for BIG1 as a potential promising therapeutic target in sepsis.

Critical roles of phosphoinositides and NF2 in Hippo pathway regulation

In this study, Hong et al. provide new insights into how NF2 mediates upstream signals to regulate the Hippo pathway. They show that NF2's lipid-binding ability is critical for its function in activating the Hippo pathway in response to osmotic stress in mammalian cells, and identify the PIP5K family as novel regulators upstream of Hippo signaling.

The Hippo pathway is a master regulator of tissue homeostasis and organ size. NF2 is a well-established tumor suppressor, and loss of NF2 severely compromises Hippo pathway activity. However, the precise mechanism of how NF2 mediates upstream signals to regulate the Hippo pathway is not clear. Here we report that, in mammalian cells, NF2's lipid-binding ability is critical for its function in activating the Hippo pathway in response to osmotic stress. Mechanistically, osmotic stress induces PI(4,5)P₂ plasma membrane enrichment by activating the PIP5K family, allowing for NF2 plasma membrane recruitment and subsequent downstream Hippo pathway activation. An NF2 mutant deficient in lipid binding is unable to activate the Hippo pathway in response to osmotic stress, as measured by LATS and YAP phosphorylation. Our findings identify the PIP5K family as novel regulators upstream of Hippo signaling, and uncover the importance of phosphoinositide dynamics, specifically PI(4,5)P₂, in Hippo pathway regulation.

A PI(4,5)P2-derived "gasoline engine model" for the sustained B cell receptor activation

To efficiently initiate activation responses against rare ligands in the microenvironment, lymphocytes employ sophisticated mechanisms involving signaling amplification. Recently, a signaling amplification mechanism initiated from phosphatidylinositol (PI) 4, 5-biphosphate [PI(4,5)P2] hydrolysis and synthesis for sustained B cell activation has been reported. Antigen and B cell receptor (BCR) recognition triggered the prompt reduction of PI(4,5)P2 density within the BCR microclusters, which led to the positive feedback for the synthesis of PI(4,5)P2 outside of the BCR microclusters. At single molecule level, the diffusion of PI(4,5)P2 was slow, allowing for the maintenance of a PI(4,5)P2 density gradient between the inside and outside of the BCR microclusters and the persistent supply of PI(4,5)P2 from outside to inside of the BCR microclusters. Here, we review studies that have contributed to uncovering the molecular mechanisms of PI(4,5)P2-derived signaling amplification model. Based on these studies, we proposed a "gasoline engine model" in which the activation of B cell signaling inside the microclusters is similar to the working principle of burning gasoline within the engine chamber of a gasoline engine. We also discuss the evidences showing the potential universality of this model and future prospects.

Small GTPase ARF6 Is a Coincidence-Detection Code for RPH3A Polarization in Neutrophil Polarization

Cell polarization is a key step for leukocytes adhesion and transmigration during leukocytes' inflammatory infiltration. Polarized localization of plasma membrane (PM) phosphatidylinositol-4-phosphate (PtdIns4P) directs the polarization of RPH3A, which contains a PtdIns4P binding site. Consequently, RPH3A mediates the RAB21 and PIP5K1C90 polarization, which is important for neutrophil adhesion to endothelia during inflammation. However, the mechanism by which RPH3A is recruited only to PM PtdIns4P rather than Golgi PtdIns4P remains unclear. By using ADP-ribosylation factor 6 (ARF6) small interfering RNA, ARF6 dominant-negative mutant ARF6(T27N), and ARF6 activation inhibitor SecinH3, we demonstrate that ARF6 plays an important role in the polarization of RPH3A, RAB21, and PIP5K1C90 in murine neutrophils. PM ARF6 is polarized and colocalized with RPH3A, RAB21, PIP5K1C90, and PM PtdIns4P in mouse and human neutrophils upon integrin stimulation. Additionally, ARF6 binds to RPH3A and enhances the interaction between the PM PtdIns4P and RPH3A. Consistent with functional roles of polarization of RPH3A, Rab21, and PIP5K1C90, ARF6 is also required for neutrophil adhesion on the inflamed endothelial layer. Our study reveals a previously unknown role of ARF6 in neutrophil polarization as being the coincidence-detection code with PM PtdIns4P. Cooperation of ARF6 and PM PtdIns4P direct RPH3A polarization, which is important for neutrophil firm adhesion to endothelia.

Phosphoinositides: Regulators of Nervous System Function in Health and Disease

Phosphoinositides, the seven phosphorylated derivatives of phosphatidylinositol have emerged as regulators of key sub-cellular processes such as membrane transport, cytoskeletal function and plasma membrane signaling in eukaryotic cells. All of these processes are also present in the cells that constitute the nervous system of animals and in this setting too, these are likely to tune key aspects of cell biology in relation to the unique structure and function of neurons. Phosphoinositides metabolism and function are mediated by enzymes and proteins that are conserved in evolution, and analysis of knockouts of these in animal models implicate this signaling system in neural function. Most recently, with the advent of human genome analysis, mutations in genes encoding components of the phosphoinositide signaling pathway have been implicated in human diseases although the cell biological basis of disease phenotypes in many cases remains unclear. In this review we evaluate existing evidence for the involvement of phosphoinositide signaling in human nervous system diseases and discuss ways of enhancing our understanding of the role of this pathway in the human nervous system's function in health and disease.

The Caenorhabditis elegans Tubby homolog dynamically modulates olfactory cilia membrane morphogenesis and phospholipid composition

Plasticity in sensory signaling is partly mediated via regulated trafficking of signaling molecules to and from primary cilia. Tubby-related proteins regulate ciliary protein transport; however, their roles in remodeling cilia properties are not fully understood. We find that the C. elegans TUB-1 Tubby homolog regulates membrane morphogenesis and signaling protein transport in specialized sensory cilia. In particular, TUB-1 is essential for sensory signaling-dependent reshaping of olfactory cilia morphology. We show that compromised sensory signaling alters cilia membrane phosphoinositide composition via TUB-1-dependent trafficking of a PIP5 kinase. TUB-1 regulates localization of this lipid kinase at the cilia base in part via localization of the AP-2 adaptor complex subunit DPY-23. Our results describe new functions for Tubby proteins in the dynamic regulation of cilia membrane lipid composition, morphology, and signaling protein content, and suggest that this conserved family of proteins plays a critical role in mediating cilia structural and functional plasticity.

The Dlg Module and Clathrin-Mediated Endocytosis Regulate EGFR Signaling and Cyst Cell-Germline Coordination in the Drosophila Testis

Tissue homeostasis and repair relies on proper communication of stem cells and their differentiating daughters with the local tissue microenvironment. In the Drosophila male germline adult stem cell lineage, germ cells proliferate and progressively differentiate enclosed in supportive somatic cyst cells, forming a small organoid, the functional unit of differentiation. Here we show that cell polarity and vesicle trafficking influence signal transduction in cyst cells, with profound effects on the germ cells they enclose. Our data suggest that the cortical components Dlg, Scrib, Lgl and the clathrin-mediated endocytic (CME) machinery downregulate epidermal growth factor receptor (EGFR) signaling. Knockdown of dlg, scrib, lgl, or CME components in cyst cells resulted in germ cell death, similar to increased signal transduction via the EGFR, while lowering EGFR or downstream signaling components rescued the defects. This work provides insights into how cell polarity and endocytosis cooperate to regulate signal transduction and sculpt developing tissues.

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Dlg, Scrib, Lgl, and clathrin-mediated endocytosis (CME) attenuate EGFR signaling

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Knockdown of Dlg module or CME results in cell non-autonomous germ cell death

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Dlg module and CME control MAPK activation and the levels of the PIP₂ phospholipid

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PIP₂ and its synthesizing kinase Sktl/dPIP5K mediate MAPK activation

Regulation of actin assembly by PI(4,5)P2 and other inositol phospholipids: An update on possible mechanisms

Actin cytoskeleton dynamics depend on a tight regulation of actin filament formation from an intracellular pool of monomers, followed by their linkage to each other or to cell membranes, followed by their depolymerization into a fresh pool of actin monomers. The ubiquitous requirement for continuous actin remodeling that is necessary for many cellular functions is orchestrated in large part by actin binding proteins whose affinity for actin is altered by inositol phospholipids, most prominently PI(4,5)P2 (phosphatidylinositol 4,5-bisphosphate). The kinetics of PI(4,5)P2 synthesis and hydrolysis, its lateral distribution within the lipid bilayer, and coincident detection of PI(4,5)P2 and another signal, all play a role in determining when and where a particular PI(4,5)P2-regulated protein is inactivated or activated to exert its effect on the actin cytoskeleton. This review summarizes a range of models that have been developed to explain how PI(4,5)P2 might function in the complex chemical and structural environment of the cell based on a combination of experiment and computational studies.

Phosphoinositide 3-kinase pathways and autophagy require phosphatidylinositol phosphate kinases

Phosphatidylinositol phosphate kinases (PIPKs) generate a lipid messenger phosphatidylinositol 4,5-bisphosphate (PI4,5P2) that controls essentially all aspects of cellular functions. PI4,5P2 rapidly diffuses in the membrane of the lipid bilayer and does not greatly change in membrane or cellular content, and thus PI4,5P2 generation by PIPKs is tightly linked to its usage in subcellular compartments. Based on this verity, recent study of PI4,5P2 signal transduction has been focused on investigations of individual PIPKs and their underlying molecular regulation of cellular processes. Here, we will discuss recent advances in the study of how PIPKs control specific cellular events through assembly and regulation of PI4,5P2 effectors that mediate specific cellular processes. A focus will be on the roles of PIPKs in control of the phosphoinositide 3-kinase pathway and autophagy.

Hepatic deletion of p110α and p85α results in insulin resistance despite sustained IRS1-associated phosphatidylinositol kinase activity

BACKGROUND

Class IA phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) is an integral mediator of insulin signaling. The p110 catalytic and p85 regulatory subunits of PI3K are the products of separate genes, and while they come together to make the active heterodimer, they have opposing roles in insulin signaling and action. Deletion of hepatic p110α results in an impaired insulin signal and severe insulin resistance, whereas deletion of hepatic p85α results in improved insulin sensitivity due to sustained levels of phosphatidylinositol (3,4,5)-trisphosphate. Here, we created mice with combined hepatic deletion of p110α and p85α (L-DKO) to study the impact on insulin signaling and whole body glucose homeostasis.

METHODS

Six-week old male flox control and L-DKO mice were studied over a period of 18 weeks, during which weight and glucose levels were monitored, and glucose tolerance tests, insulin tolerance test and pyruvate tolerance test were performed. Fasting insulin, insulin signaling mediators, PI3K activity and insulin receptor substrate (IRS)1-associated phosphatidylinositol kinase activity were examined at 10 weeks. Liver, muscle and white adipose tissue weight was recorded at 10 weeks and 25 weeks.

RESULTS

The L-DKO mice showed a blunted insulin signal downstream of PI3K, developed markedly impaired glucose tolerance, hyperinsulinemia and had decreased liver and adipose tissue weights. Surprisingly, however, these mice displayed normal hepatic glucose production, normal insulin tolerance, and intact IRS1-associated phosphatidylinositol kinase activity without compensatory upregulated signaling of other classes of PI3K.

CONCLUSIONS

The data demonstrate an unexpectedly overall mild metabolic phenotype of the L-DKO mice, suggesting that lipid kinases other than PI3Ks might partially compensate for the loss of p110α/p85α by signaling through other nodes than Akt/Protein Kinase B.

Hepatic deletion of p110α and p85α results in insulin resistance despite sustained IRS1-associated phosphatidylinositol kinase activity

Background: Class IA phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) is an integral mediator of insulin signaling. The p110 catalytic and p85 regulatory subunits of PI3K are the products of separate genes, and while they come together to make the active heterodimer, they have opposing roles in insulin signaling and action. Deletion of hepatic p110α results in an impaired insulin signal and severe insulin resistance, whereas deletion of hepatic p85α results in improved insulin sensitivity due to sustained levels of phosphatidylinositol (3,4,5)-trisphosphate. Here, we created mice with combined hepatic deletion of p110α and p85α (L-DKO) to study the impact on insulin signaling and whole body glucose homeostasis. Methods: Six-week old male flox control and L-DKO mice were studied over a period of 18 weeks, during which weight and glucose levels were monitored, and glucose tolerance tests, insulin tolerance test and pyruvate tolerance test were performed. Fasting insulin, insulin signaling mediators, PI3K activity and insulin receptor substrate (IRS)1-associated phosphatidylinositol kinase activity were examined at 10 weeks. Liver, muscle and white adipose tissue weight was recorded at 10 weeks and 25 weeks. Results: The L-DKO mice showed a blunted insulin signal downstream of PI3K, developed markedly impaired glucose tolerance, hyperinsulinemia and had decreased liver and adipose tissue weights. Surprisingly, however, these mice displayed normal hepatic glucose production, normal insulin tolerance, and intact IRS1-associated phosphatidylinositol kinase activity without compensatory upregulated signaling of other classes of PI3K. Conclusions: The data demonstrate an unexpectedly overall mild metabolic phenotype of the L-DKO mice, suggesting that lipid kinases other than PI3Ks might partially compensate for the loss of p110α/p85α by signaling through other nodes than Akt/Protein Kinase B.

Regulation of HGF-induced hepatocyte proliferation by the small GTPase Arf6 through the PIP2-producing enzyme PIP5K1A

HGF and its receptor c-Met are critical molecules in various biological processes. Others and we have previously shown that the small GTPase Arf6 plays a pivotal role in HGF signaling in hepatocytes. However, the molecular mechanism of how Arf6 regulates HGF signaling is unclear. Here, we show that Arf6 plays an important role in HGF-stimulated hepatocyte proliferation and liver regeneration through the phosphatidylinositol 4,5-bisphosphate (PIP₂)-producing enzyme PIP5K1A. We find that knockdown of Arf6 and PIP5K1A in HepG2 cells inhibits HGF-stimulated proliferation, Akt activation, and generation of phosphatidylinositol 3,4,5-trisphosphate (PIP₃) and its precursor PIP₂. Interestingly, PIP5K1A is recruited to c-Met upon HGF stimulation in an Arf6 activity-dependent manner. Finally, we show that hepatocyte proliferation and liver regeneration after partial hepatectomy are suppressed in Pip5k1a knockout mice. These results provide insight into the molecular mechanism for HGF-stimulated hepatocyte proliferation and liver regeneration: Arf6 recruits PIP5K1A to c-Met and activates it upon HGF stimulation to produce PIP₂ and subsequently PIP₃, which in turn activates Akt to promote hepatocyte proliferation, thereby accelerating liver regeneration after liver injury.

RASSF4 controls SOCE and ER-PM junctions through regulation of PI(4,5)P2

RAS association domain family 4 (RASSF4) is involved in tumorigenesis. Chen et al. show that RASSF4 regulates store-operated Ca²⁺ entry and ER–PM junctions by affecting PI(4,5)P₂ levels. RASSF4 interacts with and regulates the activity of ARF6, an upstream regulator of PIP5K and PI(4,5)P₂.

RAS association domain family 4 (RASSF4) is involved in tumorigenesis and regulation of the Hippo pathway. In this study, we identify new functional roles of RASSF4. First, we discovered that RASSF4 regulates store-operated Ca²⁺ entry (SOCE), a fundamental Ca²⁺ signaling mechanism, by affecting the translocation of the endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) to ER–plasma membrane (PM) junctions. It was further revealed that RASSF4 regulates the formation of ER–PM junctions and the ER–PM tethering function of extended synaptotagmins E-Syt2 and E-Syt3. Moreover, steady-state PM phosphatidylinositol 4,5-bisphosphate (PI[4,5]P₂) levels, important for localization of STIM1 and E-Syts at ER–PM junctions, were reduced in RASSF4-knockdown cells. Furthermore, we demonstrated that RASSF4 interacts with and regulates the activity of adenosine diphosphate ribosylation factor 6 (ARF6), a small G protein and upstream regulator of type I phosphatidylinositol phosphate kinases (PIP5Ks) and PM PI(4,5)P₂ levels. Overall, our study suggests that RASSF4 controls SOCE and ER–PM junctions through ARF6-dependent regulation of PM PI(4,5)P₂ levels, pivotal for a variety of physiological processes.

Multifactorial Regulation of G Protein-Coupled Receptor Endocytosis

Endocytosis is a process by which cells absorb extracellular materials via the inward budding of vesicles formed from the plasma membrane. Receptor-mediated endocytosis is a highly selective process where receptors with specific binding sites for extracellular molecules internalize via vesicles. G protein-coupled receptors (GPCRs) are the largest single family of plasma-membrane receptors with more than 1000 family members. But the molecular mechanisms involved in the regulation of GPCRs are believed to be highly conserved. For example, receptor phosphorylation in collaboration with β-arrestins plays major roles in desensitization and endocytosis of most GPCRs. Nevertheless, a number of subsequent studies showed that GPCR regulation, such as that by endocytosis, occurs through various pathways with a multitude of cellular components and processes. This review focused on i) functional interactions between homologous and heterologous pathways, ii) methodologies applied for determining receptor endocytosis, iii) experimental tools to determine specific endocytic routes, iv) roles of small guanosine triphosphate-binding proteins in GPCR endocytosis, and v) role of post-translational modification of the receptors in endocytosis.

Unravelling the Mechanism of TrkA-Induced Cell Death by Macropinocytosis in Medulloblastoma Daoy Cells

Macropinocytosis is a normal cellular process by which cells internalize extracellular fluids and nutrients from their environment and is one strategy that Ras-transformed pancreatic cancer cells use to increase uptake of amino acids to meet the needs of rapid growth. Paradoxically, in non-Ras transformed medulloblastoma brain tumors, we have shown that expression and activation of the receptor tyrosine kinase TrkA overactivates macropinocytosis, resulting in the catastrophic disintegration of the cell membrane and in tumor cell death. The molecular basis of this uncontrolled form of macropinocytosis has not been previously understood. Here, we demonstrate that the overactivation of macropinocytosis is caused by the simultaneous activation of two TrkA-mediated pathways: (i) inhibition of RhoB via phosphorylation at Ser(185) by casein kinase 1, which relieves actin stress fibers, and (ii) FRS2-scaffolded Src and H-Ras activation of RhoA, which stimulate actin reorganization and the formation of lamellipodia. Since catastrophic macropinocytosis results in brain tumor cell death, improved understanding of the mechanisms involved will facilitate future efforts to reprogram tumors, even those resistant to apoptosis, to die.

AIP1-mediated stress signaling in atherosclerosis and arteriosclerosis

AIP1 (ASK1-interacting protein-1; encoded by the DAB2IP gene), a signaling scaffolding protein, is abundantly expressed in vascular endothelial cells (EC). While it was initially discovered as an apoptosis signal-regulating kinase 1 (ASK1)-interacting protein, AIP1 broadly suppresses inflammatory responses triggered by cytokines and stresses such as TNF, LPS, VEGF, and endoplasmic reticulum (ER) stress in EC (therefore, AIP1 is an anti-inflammatory protein). Human genome-wide association study (GWAS) has identified DAB2IP gene variants conferring susceptibility to cardiovascular diseases. Consistently, a global or vascular EC-specific deletion of DAB2IP in mice strongly enhances inflammatory responses and exacerbates atherosclerosis and graft arteriosclerosis progression in mouse models. Mechanisms for AIP1 function and regulation associated with human cardiovascular diseases need further investigations.

The multifaceted role of PIP2 in leukocyte biology

Phosphatidylinositol 4,5-bisphosphate (PIP2) represents about 1 % of plasma membrane phospholipids and behaves as a pleiotropic regulator of a striking number of fundamental cellular processes. In recent years, an increasing body of literature has highlighted an essential role of PIP2 in multiple aspects of leukocyte biology. In this emerging picture, PIP2 is envisaged as a signalling intermediate itself and as a membrane-bound regulator and a scaffold of proteins with specific PIP2 binding domains. Indeed PIP2 plays a key role in several functions. These include directional migration in neutrophils, integrin-dependent adhesion in T lymphocytes, phagocytosis in macrophages, lysosomes secretion and trafficking at immune synapse in cytolytic effectors and secretory cells, calcium signals and gene transcription in B lymphocytes, natural killer cells and mast cells. The coordination of these different aspects relies on the spatio-temporal organisation of distinct PIP2 pools, generated by the main PIP2 generating enzyme, phosphatidylinositol 4-phosphate 5-kinase (PIP5K). Three different isoforms of PIP5K, named α, β and γ, and different splice variants have been described in leukocyte populations. The isoform-specific coupling of specific isoforms of PIP5K to different families of activating receptors, including integrins, Fc receptors, toll-like receptors and chemokine receptors, is starting to be reported. Furthermore, PIP2 is turned over by multiple metabolising enzymes including phospholipase C (PLC) γ and phosphatidylinositol 3-kinase (PI3K) which, along with Rho family small G proteins, is widely involved in strategic functions within the immune system. The interplay between PIP2, lipid-modifying enzymes and small G protein-regulated signals is also discussed.

Emerging roles of PtdIns(4,5)P2--beyond the plasma membrane

Phosphoinositides are a collection of lipid messengers that regulate most subcellular processes. Amongst the seven phosphoinositide species, the roles for phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] at the plasma membrane, such as in endocytosis, exocytosis, actin polymerization and focal adhesion assembly, have been extensively studied. Recent studies have argued for the existence of PtdIns(4,5)P2 at multiple intracellular compartments, including the nucleus, endosomes, lysosomes, autolysosomes, autophagic precursor membranes, ER, mitochondria and the Golgi complex. Although the generation, regulation and functions of PtdIns(4,5)P2 are less well-defined in most other intracellular compartments, accumulating evidence demonstrates crucial roles for PtdIns(4,5)P2 in endolysosomal trafficking, endosomal recycling, as well as autophagosomal pathways, which are the focus of this Commentary. We summarize and discuss how phosphatidylinositol phosphate kinases, PtdIns(4,5)P2 and PtdIns(4,5)P2-effectors regulate these intracellular protein and membrane trafficking events.

Correlation Between Daam2 Expression Changes and Demyelination in Guillain-Barre Syndrome

Wnt signaling has been implicated in developmental and regenerative myelination of the CNS and PNS. The present translational investigation was undertaken to assess whether a soluble factor like Wnt may be responsible for the critically important skeletal muscle neuromuscular junction-Schwann cell communication. Specifically, three key aspects were examined: (a) whether the expression of Daam2, disheveled-associated activator of morphogenesis, a key Wnt signaling downstream effector, and PIP5K is changed in the demyelinating conditions and under different stages of progress of clinical recovery of patients with Guillain-Barre syndrome; (b) whether critical protein interactions of Daam2 with disheveled and Arf6 are changed; and (c) whether expression of c-Jun/Krox, a key negative regulator of remyelination, is changed. Daam2 was elevated in acute presentation in GB syndrome. Reduction occurred with clinical improvement of the patients. With progressive clinical improvement, c-Jun/Krox expression significantly reduced with time. Wnt signaling likely causes immediate early gene activation and transcriptional shutdown of factors critical for formation and maintenance of myelination. Whether the findings of the present study are specific to pathophysiology of demyelination in acute infectious polyradiculopathy and multiple sclerosis or a generalized aspect of demyelinating diseases merits to be examined in future studies.

Phosphorylation of phosphatidylinositol 4-phosphate 5-kinase γ by Akt regulates its interaction with talin and focal adhesion dynamics

The type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K) family members and their lipid product, phosphatidylinositol 4,5-bisphosphate (PIP2) are important regulators of actin cytoskeleton. PIP5Kγ 90kDa (PIP5Kγ90), an isoform of PIP5K, localizes to focal adhesions (FAs) and is activated via its interaction with the cytoskeletal protein, talin. Currently, regulatory signaling pathways of talin-PIP5Kγ90 interaction related to FA dynamics and cell motility are not well understood. Considering the presence of Akt consensus motifs in PIP5Kγ90, we examined a potential link of Akt activation to talin-PIP5Kγ90 interaction. We found that Akt phosphorylated PIP5Kγ90 specifically at serine 555 (S555) in vitro and in epidermal growth factor (EGF)-treated cells phosphoinositide 3-kinase-dependently. EGF treatment suppressed talin-PIP5Kγ90 interaction and PIP2 levels. Similarly, a phosphomimetic mutant (S555D), but not non-phosphorylatable mutant (S555A), of PIP5Kγ90 had reduced talin binding affinity, lowered PIP2 levels, and was dislocated from FAs. The S555D mutant also caused decreases in actin stress fibers and vinculin-positive FAs. Moreover, assembly and disassembly of FAs were enhanced by S555D expression and EGF-induced cell migration was relatively low in S555A-expressing cells compared to wild-type-expressing cells. PIP5Kγ87, a PIP5Kγ splice variant lacking the talin binding motif, was phosphorylated by Akt, which, however, hardly affected PIP2 levels. Taken together, our results suggested that Akt-mediated PIP5Kγ90 S555 phosphorylation is a novel regulatory point for talin binding to control PIP2 level at the FAs, thereby modulating FA dynamics and cell motility.

Phosphoinositide turnover in Toll-like receptor signaling and trafficking

Lipid components in biological membranes are essential for maintaining cellular function. Phosphoinositides, the phosphorylated derivatives of phosphatidylinositol (PI), regulate many critical cell processes involving membrane signaling, trafficking, and reorganization. Multiple metabolic pathways including phosphoinositide kinases and phosphatases and phospholipases tightly control spatio-temporal concentration of membrane phosphoinositides. Metabolizing enzymes responsible for PI 4,5-bisphosphate (PI(4,5)P2) production or degradation play a regulatory role in Toll-like receptor (TLR) signaling and trafficking. These enzymes include PI 4-phosphate 5-kinase, phosphatase and tensin homolog, PI 3-kinase, and phospholipase C. PI(4,5)P2 mediates the interaction with target cytosolic proteins to induce their membrane translocation, regulate vesicular trafficking, and serve as a precursor for other signaling lipids. TLR activation is important for the innate immune response and is implicated in diverse pathophysiological disorders. TLR signaling is controlled by specific interactions with distinct signaling and sorting adaptors. Importantly, TLR signaling machinery is differentially formed depending on a specific membrane compartment during signaling cascades. Although detailed mechanisms remain to be fully clarified, phosphoinositide metabolism is promising for a better understanding of such spatio-temporal regulation of TLR signaling and trafficking. [BMB Reports 2014; 47(7): 361-368]

Type I phosphatidylinositol 4-phosphate 5-kinase homo- and heterodimerization determines its membrane localization and activity

Type I phosphatidylinositol 4-phosphate 5-kinases (PIP5KIs; α, β, and γ) are a family of isoenzymes that produce phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] using phosphatidylinositol 4-phosphate as substrate. Their structural homology with the class II lipid kinases [type II phosphatidylinositol 5-phosphate 4-kinase (PIP4KII)] suggests that PIP5KI dimerizes, although this has not been formally demonstrated. Neither the hypothetical structural dimerization determinants nor the functional consequences of dimerization have been studied. Here, we used Förster resonance energy transfer, coprecipitation, and ELISA to show that PIP5KIβ forms homo- and heterodimers with PIP5KIγ_i2 in vitro and in live human cells. Dimerization appears to be a general phenomenon for PIP5KI isoenzymes because PIP5KIβ/PIP5KIα heterodimers were also detected by mass spectrometry. Dimerization was independent of actin cytoskeleton remodeling and was also observed using purified proteins. Mutagenesis studies of PIP5KIβ located the dimerization motif at the N terminus, in a region homologous to that implicated in PIP4KII dimerization. PIP5KIβ mutants whose dimerization was impaired showed a severe decrease in PI(4,5)P2 production and plasma membrane delocalization, although their association to lipid monolayers was unaltered. Our results identify dimerization as an integral feature of PIP5K proteins and a central determinant of their enzyme activity.

ADP ribosylation factor 6 (ARF6) promotes acrosomal exocytosis by modulating lipid turnover and Rab3A activation

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. ARF6 (ADP-ribosylation factor 6) is a small GTPase implicated in exocytosis, but its downstream effectors remain elusive in this process. We combined biochemical, functional, and microscopy-based methods to show that ARF6 is present in human sperm, localizes to the acrosomal region, and is required for calcium and diacylglycerol-induced exocytosis. Results from pulldown assays show that ARF6 exchanges GDP for GTP in sperm challenged with different exocytic stimuli. Myristoylated and guanosine 5'-3-O-(thio)triphosphate (GTPγS)-loaded ARF6 (active form) added to permeabilized sperm induces acrosome exocytosis even in the absence of extracellular calcium. We explore the ARF6 signaling cascade that promotes secretion. We demonstrate that ARF6 stimulates a sperm phospholipase D activity to produce phosphatidic acid and boosts the synthesis of phosphatidylinositol 4,5-bisphosphate. We present direct evidence showing that active ARF6 increases phospholipase C activity, causing phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate-dependent intra-acrosomal calcium release. We show that active ARF6 increases the exchange of GDP for GTP on Rab3A, a prerequisite for secretion. We propose that exocytic stimuli activate ARF6, which is required for acrosomal calcium efflux and the assembly of the membrane fusion machinery. This report highlights the physiological importance of ARF6 as a key factor for human sperm exocytosis and fertilization.

ADP-ribosylation factor 6 (ARF6) bidirectionally regulates dendritic spine formation depending on neuronal maturation and activity

Recent studies have reported conflicting results regarding the role of ARF6 in dendritic spine development, but no clear answer for the controversy has been suggested. We found that ADP-ribosylation factor 6 (ARF6) either positively or negatively regulates dendritic spine formation depending on neuronal maturation and activity. ARF6 activation increased the spine formation in developing neurons, whereas it decreased spine density in mature neurons. Genome-wide microarray analysis revealed that ARF6 activation in each stage leads to opposite patterns of expression of a subset of genes that are involved in neuronal morphology. ARF6-mediated Rac1 activation via the phospholipase D pathway is the coincident factor in both stages, but the antagonistic RhoA pathway becomes involved in the mature stage. Furthermore, blocking neuronal activity in developing neurons using tetrodotoxin or enhancing the activity in mature neurons using picrotoxin or chemical long term potentiation reversed the effect of ARF6 on each stage. Thus, activity-dependent dynamic changes in ARF6-mediated spine structures may play a role in structural plasticity of mature neurons.

Regulatory roles of phosphoinositides in membrane trafficking and their potential impact on cell-wall synthesis and re-modelling

BACKGROUND

Plant cell walls are complex matrices of carbohydrates and proteins that control cell morphology and provide protection and rigidity for the plant body. The construction and maintenance of this intricate system involves the delivery and recycling of its components through a precise balance of endomembrane trafficking, which is controlled by a plethora of cell signalling factors. Phosphoinositides (PIs) are one class of signalling molecules with diverse roles in vesicle trafficking and cytoskeleton structure across different kingdoms. Therefore, PIs may also play an important role in the assembly of plant cell walls.

SCOPE

The eukaryotic PI pathway is an intricate network of different lipids, which appear to be divided in different pools that can partake in vesicle trafficking or signalling. Most of our current understanding of how PIs function in cell metabolism comes from yeast and mammalian systems; however, in recent years significant progress has been made towards a better understanding of the plant PI system. This review examines the current state of knowledge of how PIs regulate vesicle trafficking and their potential influence on plant cell-wall architecture. It considers first how PIs are formed in plants and then examines their role in the control of vesicle trafficking. Interactions between PIs and the actin cytoskeleton and small GTPases are also discussed. Future challenges for research are suggested.

Preferential localization of type I phosphatidylinositol 4-phosphate 5-kinase γ at the periactive zone of mouse photoreceptor ribbon synapses

Type I phosphatidylinositol 4-phosphate 5 kinase γ (PIP5KIγ) constitutes a major pathway for the generation of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) that regulates a variety of neuronal functions at both presynaptic and postsynaptic compartments. In this study, we examined the expression and localization of PIP5KIγ in the adult mouse retina. RT-PCR analysis revealed that PIP5KIγ_v2 was predominantly expressed in the retina while PIP5KIγ_v3 was also expressed faintly. Immunostaining of the adult mouse retina revealed intense PIP5KIγ-immunoreactivity in the inner and outer plexiform layers in a punctate manner. In the photoreceptor ribbon synapse, PIP5KIγ was highly concentrated at the periactive zone. These findings suggest that PIP5KIγ, especially PIP5KIγ_i2, is localized at the periactive zone, a functionally suitable compartment for the endocytosis of synaptic vesicles in photoreceptor ribbon synapses.

Steering neuronal growth cones by shifting the imbalance between exocytosis and endocytosis

Extracellular molecular cues guide migrating growth cones along specific routes during development of axon tracts. Such processes rely on asymmetric elevation of cytosolic Ca(2+) concentrations across the growth cone that mediates its attractive or repulsive turning toward or away from the side with Ca(2+) elevation, respectively. Downstream of these Ca(2+) signals, localized activation of membrane trafficking steers the growth cone bidirectionally, with endocytosis driving repulsion and exocytosis causing attraction. However, it remains unclear how Ca(2+) can differentially regulate these opposite membrane-trafficking events. Here, we show that growth cone turning depends on localized imbalance between exocytosis and endocytosis and identify Ca(2+)-dependent signaling pathways mediating such imbalance. In embryonic chicken dorsal root ganglion neurons, repulsive Ca(2+) signals promote clathrin-mediated endocytosis through a 90 kDa splice variant of phosphatidylinositol-4-phosphate 5-kinase type-1γ (PIPKIγ90). In contrast, attractive Ca(2+) signals facilitate exocytosis but suppress endocytosis via Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and cyclin-dependent kinase 5 (Cdk5) that can inactivate PIPKIγ90. Blocking CaMKII or Cdk5 leads to balanced activation of both exocytosis and endocytosis that causes straight growth cone migration even in the presence of guidance signals, whereas experimentally perturbing the balance restores the growth cone's turning response. Remarkably, the direction of this resumed turning depends on relative activities of exocytosis and endocytosis, but not on the type of guidance signals. Our results suggest that navigating growth cones can be redirected by shifting the imbalance between exocytosis and endocytosis, highlighting the importance of membrane-trafficking imbalance for axon guidance and, possibly, for polarized cell migration in general.

Syntenin-ALIX exosome biogenesis and budding into multivesicular bodies are controlled by ARF6 and PLD2

Exosomes are small vesicles that are secreted by cells and act as mediators of cell to cell communication. Because of their potential therapeutic significance, important efforts are being made towards characterizing exosomal contents. However, little is known about the mechanisms that govern exosome biogenesis. We have recently shown that the exosomal protein syntenin supports exosome production. Here we identify the small GTPase ADP ribosylation factor 6 (ARF6) and its effector phospholipase D2 (PLD2) as regulators of syntenin exosomes. ARF6 and PLD2 affect exosomes by controlling the budding of intraluminal vesicles (ILVs) into multivesicular bodies (MVBs). ARF6 also controls epidermal growth factor receptor degradation, suggesting a role in degradative MVBs. Yet ARF6 does not affect HIV-1 budding, excluding general effects on Endosomal Sorting Complexes Required for Transport. Our study highlights a novel pathway controlling ILV budding and exosome biogenesis and identifies an unexpected role for ARF6 in late endosomal trafficking.

Insights into the mechanism for dictating polarity in migrating T-cells

This review is focused on mechanisms of chemokine-induced polarization of T-lymphocytes. Polarization involves, starting from spherical cells, formation of a morphologically and functionally different rear (uropod) and front (leading edge). This polarization is required for efficient random and directed T-cell migration. The addressed topics concern the specific location of cell organelles and of receptors, signaling molecules, and cytoskeletal proteins in chemokine-stimulated polarized T-cells. In chemokine-stimulated, polarized T-cells, specific proteins, signaling molecules and organelles show enrichment either in the rear, the midzone, or the front; different from the random location in spherical resting cells. Possible mechanisms involved in this asymmetric location will be discussed. A major topic is also the functional role of proteins and cell organelles in T-cell polarization and migration. Specifically, the roles of adhesion and chemokine receptors, cytoskeletal proteins, signaling molecules, scaffolding proteins, and membrane microdomains in these processes will be discussed. The polarity which is established during contact formation of T-cells with antigen-presenting cells is not discussed in detail.

Small GTPases of the Ras superfamily regulate intestinal epithelial homeostasis and barrier function via common and unique mechanisms

The intestinal epithelium forms a stable barrier protecting underlying tissues from pathogens in the gut lumen. This is achieved by specialized integral membrane structures such as tight and adherens junctions that connect neighboring cells and provide stabilizing links to the cytoskeleton. Junctions are constantly remodeled to respond to extracellular stimuli. Assembly and disassembly of junctions is regulated by interplay of actin remodeling, endocytotic recycling of junctional proteins, and various signaling pathways. Accumulating evidence implicate small G proteins of the Ras superfamily as important signaling molecules for the regulation of epithelial junctions. They function as molecular switches circling between an inactive GDP-bound and an active GTP-bound state. Once activated, they bind different effector molecules to control cellular processes required for correct junction assembly, maintenance and remodelling. Here, we review recent advances in understanding how GTPases of the Rho, Ras, Rab and Arf families contribute to intestinal epithelial homeostasis.

Learning-related synaptic growth mediated by internalization of Aplysia cell adhesion molecule is controlled by membrane phosphatidylinositol 4,5-bisphosphate synthetic pathway

Long-term facilitation in Aplysia is accompanied by the growth of new synaptic connections between the sensory and motor neurons of the gill-withdrawal reflex. One of the initial steps leading to the growth of these synapses is the internalization, induced by 5-HT, of the transmembrane isoform of Aplysia cell-adhesion molecule (TM-apCAM) from the plasma membrane of sensory neurons (Bailey et al., 1992). However, the mechanisms that govern the internalization of TM-apCAM and how this internalization is coupled to the molecular events that initiate the structural changes are not fully understood. Here, we report that the synthesis of membrane phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], which is known to be mediated by a signaling cascade through Aplysia Sec7 protein (ApSec7) and phosphatidylinositol-4-phosphate 5-kinase type I α (PIP5KIα) is required for both the internalization of TM-apCAM and the initiation of synaptic growth during 5-HT-induced long-term facilitation. Pharmacological blockade of PI(4,5)P(2) synthesis by the application of the inhibitor phenylarsine oxide blocked the internalization of apCAM. Furthermore, perturbation of the endogenous activation of ApSec7 and its downstream target PIP5KIα also blocked 5-HT-mediated internalization of TM-apCAM and synaptic growth. Finally, long-term facilitation was specifically impaired by blocking the ApSec7 signaling pathway at sensory-to-motor neuron synapses. These data indicate that the ApSec7/PIP5KIα signaling pathway is actively recruited during learning-related 5-HT signaling and acts as a key regulator of apCAM internalization associated with the formation of new synaptic connections during long-term facilitation.

Interplay between Rab35 and Arf6 controls cargo recycling to coordinate cell adhesion and migration

Cells inversely adjust the plasma membrane levels of integrins and cadherins during cell migration and cell-cell adhesion but the regulatory mechanisms that coordinate these trafficking events remain unknown. Here, we demonstrate that the small GTPase Rab35 maintains cadherins at the cell surface to promote cell-cell adhesion. Simultaneously, Rab35 supresses the activity of the GTPase Arf6 to downregulate an Arf6-dependent recycling pathway for β1-integrin and EGF receptors, resulting in inhibition of cell migration and attenuation of signaling downstream of these receptors. Importantly, the phenotypes of decreased cell adhesion and increased cell migration observed following Rab35 knock down are consistent with the epithelial-mesenchymal transition, a feature of invasive cancer cells, and we show that Rab35 expression is suppressed in a subset of cancers characterized by Arf6 hyperactivity. Our data thus identify a key molecular mechanism that efficiently coordinates the inverse intracellular sorting and cell surface levels of cadherin and integrin receptors for cell migration and differentiation.

Emerging major synaptic signaling pathways involved in intellectual disability

Genetic causes of intellectual disability (ID) include mutations in proteins with various functions. However, many of these proteins are enriched in synapses and recent investigations point out their crucial role in the subtle regulation of synaptic activity and dendritic spine morphogenesis. Moreover, in addition to genetic data, functional and animal model studies are providing compelling evidence that supports the emerging unifying synapse-based theory for cognitive deficit. In this review, we highlight ID-related gene products involved in synaptic morphogenesis and function, with a particular focus on the emergent signaling pathways involved in synaptic plasticity whose disruption results in cognitive deficit.

Coupling exo- and endocytosis: an essential role for PIP₂ at the synapse

Chemical synapses are specialist points of contact between two neurons, where information transfer takes place. Communication occurs through the release of neurotransmitter substances from small synaptic vesicles in the presynaptic terminal, which fuse with the presynaptic plasma membrane in response to neuronal stimulation. However, as neurons in the central nervous system typically only possess ~200 vesicles, high levels of release would quickly lead to a depletion in the number of vesicles, as well as leading to an increase in the area of the presynaptic plasma membrane (and possible misalignment with postsynaptic structures). Hence, synaptic vesicle fusion is tightly coupled to a local recycling of synaptic vesicles. For a long time, however, the exact molecular mechanisms coupling fusion and subsequent recycling remained unclear. Recent work now indicates a unique role for the plasma membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)), acting together with the vesicular protein synaptotagmin, in coupling these two processes. In this work, we review the evidence for such a mechanism and discuss both the possible advantages and disadvantages for vesicle recycling (and hence signal transduction) in the nervous system. This article is part of a Special Issue entitled Lipids and Vesicular Transport.